A laser beam generator usually outputs a multi-mode laser beam including plural modes ranging from a low-order mode to a high-order mode. Depending on a purpose thereof, a laser beam having strong intensity at particular mode or modes may be required. For instance, a laser beam in which only a basic mode has strong intensity and higher-order modes have low intensity may be required. When the basic mode is dominant over other modes of a laser beam, designs and settings of an optical system that processes the laser beam can be simplified. Further, it becomes possible to increase light intensity per unit area by narrowing down a light condenser diameter. Various technical fields such as laser microfabrication processes, laser measurements, and optical communications require a laser beam in which only particular mode or modes of particular order range are dominant.
In order to obtain a basic mode-dominant laser beam from a multi-mode laser beam that includes plural modes ranging from a basic mode to higher-order modes, an iris is inserted on a propagation path of the laser beam to cut modes other than the basic mode. FIG. 1 explains this method. A semiconductor laser device 2 emits excitation beam 4, and an oscillation gain medium 6 is excited by the excitation beam 4 to output laser beam 8. Laser beam 8 includes laser beam 8a of a basic mode, and aside therefrom, it further includes laser beam 86b of a high-order mode of which order is two or more. The laser beam 8a in the basic mode emitted from the oscillation gain medium 6 progresses without hardly expanding, whereas the laser beam 8b in the high-order mode progresses while expanding. An iris 10 includes an aperture that allows the laser beam progressing without expanding to pass, and prohibits the laser beam progressing while expanding from passing. The laser beam 86b in the high-order mode progresses while expanding, and as such, it does not pass through the aperture. Laser beam 8c that had passed through the aperture is the laser beam of the basic mode, and the basic mode-dominant laser beam can be obtained. In the description herein, obtaining a laser beam in which a particular mode or modes of particular order range are dominant from a multi-mode laser beam including plural modes ranging from the basic mode to higher-order modes will be termed a mode cleaning. As shown in FIG. 1, cleaning for the basic mode can be performed by using the iris 10 provided with the aperture. However, according to this method, diffracted light 8d is undesirably generated around the laser beam 8c that had passed the aperture. Since diffracted light 8d cannot be cleaned, this procedure is not sufficient to “mode cleaning”. Depictions given reference signs 12, 14, and 16 show light contrast patterns that are achieved by observing a lateral cross-sectional shape of the laser beam at their corresponding positions.
According to an optical system of FIG. 2, sufficient mode cleaning can be performed while suppressing generation of diffracted light. In the optical system of FIG. 2, a condenser lens 18 and a collimate lens 20 are added. The generation of the diffracted light can be avoided by using a combination of the condenser lens 18, the iris 10, and the collimate lens 20 to facilitate Fourier transform process (image relay). However, in this technique, air breakdown occurs at a light condensing point when strong laser beam is used. In order to prevent the air breakdown from occurring at the light condensing point with such high-intensity laser beam, the light condensing point needs to be situated in a vacuum environment, so a vacuum container 24 provided with windows 22, 26 becomes necessary. In the method of FIG. 2, even if the laser beam generator is made compact, the vacuum container 24 exceeding the size thereof becomes necessary in addition, so the advantage of compacting the laser beam generator cannot be enjoyed. Depictions given reference signs 12, 28 show the light contrast patterns that are achieved by observing the lateral cross-sectional shape of the laser beam at their corresponding positions, and a reference sign 30 shows the basic mode-dominant laser beam that has been subjected to mode cleaning.
U.S. Pat. No. 5,818,630 describes a technique that performs mode cleaning using a mode filter such as a fiber taper or a fiber coil. The technique of U.S. Pat. No. 5,818,630 requires special fibers such as the fiber taper or the fiber coil.